This invention relates to the partial oxidation of hydrocarbons, and more particularly to the production of hydrogen and carbon monoxide by the oxidation of hydrocarbons. Specifically, the invention relates to a process comprising contacting ceramic oxygen-selective mixed conductors with steam and/or carbon dioxide at high temperatures, thereby causing oxygen to be adsorbed by the mixed conductor, and subsequently producing hydrogen and carbon monoxide by contacting the oxygen-containing mixed conductor with hydrocarbons.
Syngas and its components, hydrogen and carbon monoxide, are conventionally produced by the high temperature partial oxidation of hydrocarbons with controlled amounts of air or oxygen. Although air is less expensive and more convenient to use in partial oxidation reactions, it is less attractive than oxygen for such reactions because the large quantities of nitrogen that are produced when air is used as the oxidant must be subsequently separated from the product gas prior to its use. The cost of separation, purification and heat exchange equipment for product gas purification and recovery of heat energy from the nitrogen adds considerably to the cost of syngas production using air.
Although oxygen is more desirable than air as an oxidant for partial oxidation reactions, its use is not without disadvantage, in that oxygen must be imported into the system, or it must be generated on site, for example, by means of a cryogenic air separation plant or an adsorption system. In either alternative, using oxygen as the oxidant likewise adds considerably to the cost of the process.
On site production of oxygen using ceramic-based materials for applications such as hydrocarbon partial oxidation reactions has been recently reported. U.S. Pat. No. 5,714,091 discloses an oxygen-based hydrocarbon partial oxidation process in which the oxygen is produced on site by subjecting air to membrane separation using a membrane constructed of perovskite-type ceramic material. Oxygen, which is permeable to the membrane, passes through the membrane and is made to react with hydrocarbons on the downstream side of the membrane unit. The disadvantages of this method of oxygen production are the high cost of production of the membrane and the difficulty of producing membrane structures that are leak-proof.
The partial oxidation of hydrocarbons with oxygen retained in ceramic-based oxygen-selective mixed conducting substances, such as perovskite-type ceramics, is disclosed in copending U.S. patent application Ser. No. 09/175,175, filed Oct. 20, 1998, and Ser. No. 09/290,768, filed Apr. 13, 1999, the specifications of which are incorporated herein by reference.
In the above-described hydrocarbon partial oxidation processes, up to one-half mole of hydrogen and up to one mole of carbon monoxide, respectively, are produced for each hydrogen atom and each carbon atom contained in the hydrocarbon feed to the process. For instance, when methane is partially oxidized by the above-described processes, a maximum of two moles of hydrogen and one mole of carbon monoxide can be obtained for each molecule of methane. The equation for this reaction is:
CH4+1/202xe2x86x92CO+2H2
The present invention provides a hydrocarbon partial oxidation process which uses an oxygen-selective mixed conductor, but which has the advantage over the above-described processes of producing more hydrogen and/or more carbon monoxide for each mole of hydrocarbon feed to the process.
According to a broad embodiment, the invention comprises a process comprising the steps:
(a) contacting at least one oxygen ion-conducting ceramic with a feed gas comprising a component selected from the group consisting of steam, carbon dioxide, sulfur oxides, nitrogen oxides and mixtures thereof in an adsorption zone at a temperature in the range of about 300 to about 1400xc2x0 C. and at an absolute pressure in the range of about 0.5 to about 50 bara, thereby at least partially saturating the at least one oxygen ion-conducting ceramic with oxygen and producing hydrogen, carbon monoxide, sulfur, nitrogen or mixtures thereof; and
(b) removing oxygen from the at least partially oxygen saturated oxygen ion-conducting ceramic.
In a preferred embodiment, the at least one oxygen ion-conducting ceramic comprises an oxygen-selective mixed conductor. In a more preferred embodiment, the oxygen ion-conducting ceramic comprises a perovskite-type ceramic having the structural formula A1-xMxBO3-xcex4, where A is an ion of a metal of Groups 3a and 3b of the periodic table of elements or mixtures thereof; M is an ion of a metal of Groups 1a and 2a of the periodic table or mixtures thereof; B is an ion of a d-block transition metal of the periodic table or mixtures thereof; x varies from  greater than 0 to 1; and xcex4 is the deviation from stoichiometric composition resulting from the substitution of ions of metals of M for ions of metals of A.
In another preferred embodiment, the oxygen is removed from the at least partially saturated mixed conductor by increasing the temperature in the adsorption zone, by decreasing the pressure in the adsorption zone, by contacting the at least partially oxygen saturated mixed conductor with a reducing agent or by combinations thereof
In another preferred embodiment, the feed gas of step (a) is steam, carbon dioxide or mixtures thereof. In this preferred embodiment, step (b) of the process preferably comprises contacting the at least partially oxygen-saturated oxygen ion-conducting ceramic mixed conductor with a reducing agent comprising at least one organic compound selected from the group consisting of hydrocarbons, oxygen-containing hydrocarbons and mixtures thereof in a reaction zone at a temperature in the range of about 300 to about 1,400xc2x0 C., thereby partially oxidizing the at least one organic compound and producing product gas comprising hydrogen, carbon monoxide or mixtures of these, and at least partially depleting the mixed conductor of oxygen.
In another preferred embodiment, the process is carried out by repeatedly performing steps (a) and (b) in sequence. In one preferred aspect, the process is carried out in a fixed bed comprising the at least one oxygen-selective mixed conductor, and the fixed bed serves as the adsorption zone during step (a) and as the reaction zone during step (b). In another preferred aspect, the process is carried out in a moving bed system, and it further comprises recycling the at least partially oxygen-depleted mixed conductor to the adsorption zone. Preferably, the moving bed system is a fluidized bed system and the at least partially oxygen-saturated mixed conductor is fluidized and carried into the reaction zone by the at least one organic compound, steam, carbon dioxide or mixtures thereof.
In a more preferred embodiment, the at least one oxygen ion-conducting ceramic is a perovskite-type ceramic and x varies from about 0.1 to 1.
In another more preferred embodiment, the at least one oxygen ion-conducting ceramic is a perovskite-type ceramic and A is one or more f-block lanthanides. In a more preferred embodiment, A is La, Y, Sm or mixtures thereof.
In another more preferred embodiment, the at least one oxygen ion-conducting ceramic is a perovskite-type ceramic and M is at least one metal of Group 2a of the periodic table of elements. In a more preferred embodiment M is Sr, Ca, Ba or mixtures thereof.
In another more preferred embodiment, the at least one oxygen ion-conducting ceramic is a perovskite-type ceramic and B is Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn or mixtures thereof. In a more preferred embodiment, B is V, Fe, Ni, Cu or mixtures thereof.
In another more preferred embodiment, the at least one oxygen ion-conducting ceramic is a perovskite-type ceramic and x is about 0.2 to 1.
In another more preferred embodiment, the at least one oxygen ion-conducting ceramic is a perovskite-type ceramic and A is La, Y, Sm or mixtures thereof, M is Sr, Ca or mixtures thereof, and B is V, Fe, Ni, Cu or mixtures thereof.
In another embodiment, the at least one oxygen ion-conducting ceramic conductor is a member selected from the group consisting of (1) ceramic substances selected from the group consisting of Bi2O3, ZrO2, CeO2, ThO2, HfO2 and mixtures thereof, the ceramic substances being doped with CaO, rare earth metal oxides or mixtures of these; (2) brownmillerite oxides; and (3) mixtures of these.
In another embodiment, the at least one oxygen ion-conducting ceramic conductor is at least one ceramic substance selected from the group consisting of Bi2O3, ZrO2, CeO2, ThO2, HfO2 and mixtures of these, and the at least one ceramic substance is doped with a rare earth metal oxide selected from the group consisting of Y2O3, Nb2O3, Sm2O3, Gd2O3 and mixtures of these.
In another preferred embodiment, the process further comprises, after step (b), the additional step comprising stripping the at least partially oxygen-depleted mixed conductor with gas that is compatible with the product gas. In a more preferred embodiment, the stripping gas is steam, nitrogen, carbon dioxide or mixtures of these.
In another preferred embodiment, steps (a) and (b) of the process are carried out at a temperature in the range of about 500 to about 1,200xc2x0 C. In another preferred embodiment, step (a) is carried out at an absolute pressure in the range of about 0.5 to 20 bara. In a more preferred embodiment, steps (a) and (b) are carried out at a temperature in the range of about 650 to about 1,100xc2x0 C.
In another preferred embodiment, the at least one organic compound has an aliphatic, cycloaliphatic or aromatic structure and it contains 1 to 12 carbon atoms.
In another preferred embodiment, the at least one organic compound is aliphatic and contains 1 to 6 carbon atoms. In a more preferred embodiment, the at least one organic compound comprises methane, methanol, natural gas, at least one petroleum derivative or mixtures thereof.
In another preferred embodiment, the at least one organic compound comprises a petroleum derivative comprising naphtha, gasoline or mixtures thereof.
In another preferred embodiment, at least one agent which promotes the oxygen adsorption of step (a) and/or the partial oxidation of step (b), when step (b) is a partial oxidation step, is combined with the at least one oxygen ion-conducting ceramic. Preferably, the at least one agent is a transition metal. More preferably, the transition metal is Cu, Ag, Fe, Ni, Rh, Pt or mixtures thereof.
In another preferred embodiment, the at least one oxygen ion-conducting ceramic additionally contains a catalyst selective for partial oxidation reactions that produce partial oxidation reaction products other than hydrogen and carbon monoxide.
In another preferred embodiment, the process further comprises, during step (b), passing a moderating agent selected from steam, carbon dioxide and mixtures thereof through the at least one reaction zone. In a more preferred embodiment, the moderating agent is steam.
In another preferred embodiment, the feed gas of step (a) comprises the exhaust gas of a combustion process.
In another preferred embodiment, the process further comprises introducing air, oxygen-enriched air or substantially pure oxygen into the adsorption zone during step (a) or into the reaction zone during step (b), or into the adsorption zone and/or the reaction zone between steps (a) and (b).